1. Field of the Invention
The present invention relates to a semiconductor device, in particular, to a semiconductor device having a Schottky barrier diode structure.
2. Description of Related Art
While a Schottky barrier diode (SBD) has a low forward voltage and a high switching speed, the SBD has high reverse leakage current and a low reverse avalanche breakdown voltage as its disadvantage. In a medium or high voltage product of 100 V or higher, in particular, the reverse leakage current needs to be reduced to prevent thermorunaway. The impurity concentration of a drift region needs to be lowered so that a depletion layer is expanded while maintaining a high Schottky barrier height. As a result, forward characteristics are deteriorated.
FIG. 10 shows a structure for reducing leakage current by utilizing a pinch-off effect due to a junction used in a low voltage product. In this figure, an n-type semiconductor layer 2 (hereinafter, referred to as Nxe2x88x92 epi-layer 2) having a low impurity concentration is formed on an n-type semiconductor substrate 1 (hereinafter, referred to as n-type substrate 1) having a high impurity concentration by epitaxial growth. P-type semiconductor regions 19 (hereinafter, referred to as P+ regions 19) having a high impurity concentration are disposed at prescribed intervals in the main surface of the Nxe2x88x92 epi-layer 2 by diffusion or embedding of polycrystalline silicon in the trench. An anode electrode film 8 contacted to the main surface 6 of the Nxe2x88x92 epi-layer 2 and surfaces 20 of the P+ regions 19 is formed. The anode electrode film 8 has Schottky contact with the main surface 6 of the Nxe2x88x92epi-layer 2. A cathode electrode film 9 having ohmic contact with the n-type substrate 1 is formed on the other surface of the n-type substrate 1.
When a reverse voltage is gradually applied to an SBD in FIG. 10, depletion layers 231 extend from side surfaces 21, 22 of the P+ regions 19 into a region 2a in the Nxe2x88x92 epi-layer 2 interposed between the adjacent P+ regions 19 as shown in FIG. 11. When the reverse voltage is further applied, edges of the depletion layers extending from the side surfaces 21, 22 of the P+ regions 19 come into contact with each other (pinch-off) and become one wide depletion layer 232. Therefore, an electric field applied to an interface between the main surface 6 of the Nxe2x88x92 epi-layer 2 and the anode electrode film 8 is relaxed and thereby reverse leakage current can be reduced.
FIG. 12 shows a distribution of electric field strengths in a vertical direction along lines A and B, which are located at the centers of the P+ region 19 and the Nxe2x88x92 epi-region 2a interposed between the P+ regions 19, respectively, when a reverse avalanche breakdown voltage is applied to the semiconductor device in a pinch-off state in FIG. 10. As described above, it is evident from FIG. 12 that an electric field applied to the interface between the main surface 6 of the Nxe2x88x92 epi-layer 2 and the anode electrode film 8 is relaxed.
However, if the structure shown in FIG. 10 is applied to a medium or high voltage product of 100 V or higher, an electric field is increased at a pn junction between the bottom 24 of the P+ region 19 and the Nxe2x88x92 epi-layer 2, leading to deterioration of the reverse avalanche breakdown voltage. To maintain the reverse avalanche breakdown voltage, the impurity concentration of the Nxe2x88x92 epi-layer 2 needs to be lowered, resulting in deterioration of forward characteristics.
There is also a structure in which only a bottom portion of the P+ region 19 in FIG. 10 is formed of a p-type-semiconductor region 25 having a low impurity concentration to relax the electric field at the bottom of the P+ region 19 as shown in FIG. 13. However, if the impurity concentration of the p-type semiconductor region 25 becomes lower than a desired concentration, an electric field is concentrated at the bottom of the P+ region 19, resulting in deterioration of the reverse avalanche breakdown voltage. If the impurity concentration of the p-type semiconductor region 25 is higher than a desired concentration, an electric field is concentrated at the bottom of the p-type semiconductor region 25, also resulting in deterioration of the reverse avalanche breakdown voltage.
In this structure, a large region having a low impurity concentration needs to be provided at the bottom to sufficiently relax the electric field. However, if a region having a low impurity concentration is further extended to under the p-type semiconductor region 25, a thickness of the Nxe2x88x920 epi-layer 2 needs to be increased, resulting in deterioration of forward characteristics, which is a trade-off.
When reverse leakage current is reduced and a reverse avalanche breakdown voltage is maintained in a Schottky barrier diode with a medium or high avalanche breakdown voltage of 100 V or higher to prevent thermorunaway, there exists a trade-off that forward characteristics are deteriorated since the impurity concentration of the Nxe2x88x92 epi-layer is lowered or a pinch-off effect due to the junction is utilized.
In view of the foregoing, an object of the present invention is to provide a semiconductor device such as a Schottky barrier diode or the like in which reverse leakage current is maintained at a conventional level while forward characteristics are greatly improved.
To achieve the above object, the present invention provides a semiconductor device having a first semiconductor layer composed of a semiconductor of a first conductivity type, a second semiconductor layer of the first conductivity type having a lower impurity concentration than that of the first semiconductor layer, trench portions composed of thin trenches having a prescribed width and prescribed intervals therebetween formed in the second semiconductor layer surface, a semiconductor filled material composed of semiconductor of a second conductivity type, which is opposite to the first conductivity type, filled in the trench portions, a Schottky metal electrode formed, on the surface of the second semiconductor layer and the surface of the semiconductor filled material while forming a Schottky junction with the second semiconductor layer and an ohmic contact with the semiconductor filled material, and an ohmic metal electrode formed on the surface of the first semiconductor layer. In this semiconductor device, at least the second semiconductor layer and the semiconductor filled material are constituted by the same semiconductor material. When an avalanche breakdown voltage BVAK between the semiconductor filled material and the second semiconductor layer is expressed by
BVAK=60xc3x97(Eg/1.1)1.5xc3x97(Nd/1016)xe2x88x92xc2xe
(where the unit of BVAK is V; Nd represents an impurity concentration of the second semiconductor layer and its unit is cmxe2x88x923; and Eg represents an energy band gap value of the semiconductor material and its unit is eV), the width Wm between the semiconductor filled materials adjacent to each other formed in the second semiconductor layers satisfies the following equations (1) and (2):
(where the unit of the width Wm is cm; Wt represents a width                               W          m                ≃                                            W              t                        xc3x97                          N              d                                            N            a                                              (        1        )            
of the semiconductor filled material and its unit is cm; and Na represents an impurity concentration in the semiconductor filled material and its unit is cmxe2x88x923)                               W          m                ≦                                            2              xc3x97                              ϵ                0                            xc3x97                              ϵ                S                            xc3x97                              (                                                      BV                    AK                                    /                  n                                )                                                    q              xc3x97                              N                d                                                                        (        2        )            
(where xcex5s represents a relative permittivity of the semiconductor material; xcex50 represents a permittivity in vacuum and is 8.85418xc3x9710xe2x88x9214 F/cm; and q represents an elementary electrical charge and is 1.60218xc3x9710xe2x88x9219 coulomb; and in equation (2), n greater than 1).
The present invention also provides a semiconductor device in which an insulating film is formed on the sidewall and the bottom surface of the trench portion and the insulating film is disposed between the semiconductor filled material and the second semiconductor layer.
The present invention also provides a semiconductor device in which a high impurity concentration layer of the second conductivity type is formed on the surface of the semiconductor filled material.